1. Field of the Invention
This invention relates to a toner, and a fixing method improved in fixing performance, making use of a heating unit of an electromagnetic-induction heating system.
2. Related Background Art
In image-forming apparatus, assemblies of a heat roller system have widely been used as fixing assemblies by means of which unfixed images (toner images) of intended image information, formed and held on recording mediums (such as transfer material sheets, electrofascimile sheets, electrostatic recording paper, OHP sheets, printing sheets and format sheets) by a transfer system or direct system at an appropriate site for carrying out an image-forming process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process are heat-fixed as permanent fixed images to the surface of the recording medium. Nowadays, from the viewpoint of quick start and energy saving, assemblies of a belt or film heating system have put into practical use. Assemblies of an electromagnetic-induction heating system are also proposed. Fixing assembles of these systems have device construction and advantages or disadvantages as described below.
a) Fixing Assembly of Heat Roller System:
This is an assembly constituted basically of paired pressure contact rollers of a fixing roller (heat roller) and a pressure roller. The paired rollers are rotated, and a recording medium on which unfixed toner images to be imagewise fixed have been formed and held is guided to, and held tight at, a fixing nip which is a zone of mutual pressure contact of the paired rollers, where the unfixed toner images are fixed by heat and pressure to the recording medium surface by the action of the heat of the fixing roller and the pressing force at the fixing nip.
The fixing roller commonly comprises as a substrate (mandrel) a hollow metal roller made of aluminum, and as a heat source a halogen lamp inserted into the former's internal space. It is heated by the heat the halogen lamp generates, and is temperature-controlled by controlling electrification to the halogen lamp so that its peripheral surface can be maintained at a preset fixing temperature.
In particular, in a fixing assembly of an image-forming apparatus for forming full-color images, which is required to have the ability to sufficiently heat and melt toner image layers which are four layers at the maximum, a material having a high heat capacity is used as the mandrel of the fixing roller and a rubber elastic layer for enveloping the toner images to melt them uniformly is provided on the periphery of the mandrel. The toner images are heated through the rubber elastic layer. In some construction, a heat source is also provided in the interior of the pressure roller so that the pressure roller may also be heated and temperature-controlled.
In the fixing assembly of such a heat roller system, however, even when the power source of the image-forming apparatus is switched on and at the same time the halogen lamp, the heat source of the fixing assembly, is started to be electrified, the fixing roller requires so large a heat capacity that a considerable wait time is taken until the temperature of the assembly rises to a preset fixable temperature from the time the fixing roller stands cold entirely. Thus, this fixing assembly lacks in quick-start performance. Also, it is necessary to electrify the halogen lamp to maintain the fixing roller to a prescribed temperature-controlled state so that the action of image formation can be taken at any time also when the image-forming apparatus is kept stand-by (during non-image-formation). Thus, there has been a problem such that it requires a large power consumption.
In addition, in a fixing assembly making use of a fixing roller requiring an especially large heat capacity as in the case of the fixing assembly of the full-color image-forming apparatus, a delay may occur between temperature control and fixing roller surface temperature rise to cause problems such as faulty fixing, non-uniform gloss and offset.
b) Fixing Assembly of Film Heating System:
The fixing assembly of a film heating system is disclosed in, e.g., Japanese Patent Applications Laid-open No. 63-313182, No. 2-157878, No. 4-44075 and No. 4-204980.
More specifically, a heat-resistant film (fixing film) is held commonly between a ceramic heater as a heating element and a pressure roller as a pressure member to form a nip between them. A recording medium on which unfixed toner images to be imagewise fixed have been formed and held is guided to the zone between the film and the pressure roller at the nip, and held tight and transported together with the film so that the heat of the ceramic heater is imparted to the recording medium at the nip via the film and the unfixed toner images are fixed by heat and pressure to the recording medium surface by the aid of the pressing force at the nip.
The fixing assembly of such a film heating system can be constructed as an on-demand type assembly by using low-heat-capacity members as the ceramic heater and the film. The ceramic heater as a heat source may be electrified only when the image-forming apparatus performs image formation, to bring it into a condition where the heat has been generated at a stated fixing temperature. Thus, this fixing assembly has such an advantage that the wait time from switching on the power source of the image-forming apparatus up to a state the image formation can be performed is short (quick-start performance) and the power consumption at the standby time is greatly small (power saving).
However, as a fixing assembly for full-color image-forming apparatus and high-speed type machines, there is a difficulty in respect of the quantity of heat.
c) Fixing Assembly of Electromagnetic-induction Heating System:
Japanese Utility Model Application Laid-open No. 51-109739 discloses an induction heating fixing assembly in which electric current is induced to a fixing roller by a magnetic field to generate heat by the Joule effect. This can make the fixing roller generate heat directly by utilizing the generation of induced current, and accomplishes a fixing process which is more highly efficient than the fixing assembly of a heat roller system employing the halogen lamp as a heat source.
However, this assembly involves a great heat loss by heat dissipation because the energy of alternating magnetic field that has been induced by exciting coils serving as a magnetic-field induction means is used to heat the whole fixing roller, and has such a disadvantage that the density of fixing energy with respect to the applied energy is so low as to bring about a poor efficiency.
Accordingly, in order to obtain at a high density the energy acting on fixing, the exciting coil are set close to the fixing roller which is a heating element, or the alternating magnetic field distribution attributable to the exciting coils is concentrated in the vicinity of the fixing nip. Thus, a high-efficiency fixing assembly has been contrived.
Meanwhile, with regard to toners, e.g., black toners for commonly available black and white copying machines, a relatively highly crystallizable wax as typified by polyethylene wax or polypropylene wax is used as a release agent in order to improve properties resistant to high-temperature offset at the time of fixing. For example, such toners are disclosed in Japanese Patent Publications No. 52-3304 and No. 52-3305 and Japanese Patent Application Laid-open No. 57-52574. However, in the case of toners for full-color images, their transparency may be impaired where the full-color images are projected by an OHP (overhead projector). This is due to a high crystallizability of the release agent itself and a difference in refractive index between the release agent and materials of an OHP sheet. As a result, the projected images come to have low chroma and brightness.
To solve such a problem, as disclosed in Japanese Patent Applications Laid-open No. 4-149559 and No. 4-107467, a method is proposed in which a nucleating agent is used in combination with a wax to lower the crystallizability of the wax.
A method is further proposed in which a wax having a low crystallizability is used, as disclosed in Japanese Patent Applications Laid-open No. 4-301853 and No. 5-61238. As waxes having a relatively good transparency and a low melting point, montan waxes are available. Use of such montan waxes is disclosed in Japanese Patent Applications Laid-open No. 1-185660, No. 1-185661, No. 1-185662, No. 1-185663 and No. 1-238672.
These waxes, however, are not those which sufficiently satisfy all the transparency in OHP and the low-temperature fixing performance and high-temperature anti-offset properties at the time of heat-and-pressure fixing. Accordingly, in usual color toners, the release agent is not added so far as possible and instead an oil such as silicone oil or fluorine oil is applied to the heat fixing roller so that the high-temperature anti-offset properties can be improved and the transparency in OHP can be achieved.
However, as to the fixed images thus obtained, excess oil stays attached to their surfaces. Such oil may adhere to the photosensitive member to cause contamination or the oil may swell the fixing roller to shorten the lifetime of the fixing roller.
In addition, in order not to cause any oil streaks on the fixed images, the oil must be fed to the fixing roller surface uniformly and at a constant rate. This tends to make the fixing assembly have a large size.
Accordingly, in a heat-and-pressure fixing means in which any oil for preventing high-temperature offset is not used or such oil is used in a small quantity, it is long-awaited to provide a toner having been kept from occurrence of offset and also having good secondary-color color mixing performance, having a broad color reproduction range and promising a superior transparency of fixed images.